1. Field of the Invention
The present invention relates to a slicing signal generator, and more particularly to a slicing signal generator providing an adaptive slicing signal.
2. Description of the Related Art
Teletext carried by a TV/video signal at a VBI (vertical blanking interval) has been popularly used in TV broadcasts to provide real-time information such as weather, advertising, movie and flight schedules. To decode the teletext data carried by the TV/video signal, a slicing signal is applied. The slicing signal provides a slicing level to be compared with the TV/video signal. When the TV/video signal exceeds the slicing level, the data carried by the TV/video signal is determined as logic 1, and when the TV/video signal does not exceed the slicing level, the data carried by the TV/video signal is determined as logic 0. FIG. 1 illustrates a waveform and corresponding slicing result of a TV signal. The VBI can be divided into clock-run-in, start code (not shown in FIG. 1), and teletext data. The slicing level is typically determined by the signal amplitude during the clock-run-in interval, for example, the slicing level is the average amplitude of the TV signal received during clock-run-in interval. FIG. 2 is a block diagram of a slicing signal generator. The clock-run-in window generates an enable signal to turn on the gate 21 for the clock-run-in interval. When the gate 21 is turned on, the TV signal is passed to an average computer unit 22 to compute a slicing signal. When the TV signal is distorted by noise, the teletext data carried by the TV signal may be incorrectly decoded according to the slicing signal derived from the clock-run-in interval. FIG. 3 is a waveform of a TV signal with noise. In FIG. 3, the DC (direct current) component of the TV signal varies significantly while delivering the actual teletex data, thus the slicing signal derived from the clock-run-in interval is inappropriate.